This invention relates to a process for preparing hydrophobic finely divided particles of oxides of metals and oxides of silicon by chemically bonding hydrocarbon radicals to the surface of the oxide particles. More particularly, the oxides are treated with hydrolysable metal or metalloid compounds containing OR groups.
It is known in the art that particles can be rendered hydrophobic by treatment with organo -silanes to form metals or metalloids on the surface of the particles. Such products are particularly useful when the hydrocarbon radicals (preferably methyl) are chemically bonded to the surface of finely distributed oxides, which renders the oxides, depending upon the quantity of hydrocarbon radicals present, either partially or completely hydrophobic.
It is characteristic of this type of treated particle that the transition from entirely hydrophilic to entirely hydrophobic behavior first occurs with a certain "degree of hydrophobing", more or less immediately without any distinct intermediate phase. Thus for example, products of this type with only 0.3% C or less possess hydrophilic characteristics and behave like the untreated materials. On the other hand, products with at least about 0.5 - 0.7% C are practically entirely hydrophobic depending on the specific surface characteristics of the starting oxides. It would be desirable to be able to prepare products having a degree of hydrophobing lying between these extremes, which behave like mixtures of hydrophobic and hydrophilic oxides. However, experience has shown that it is not possible to obtain products which are hydrophobic and cannot be wetted with water, but which nevertheless retain certain hydrophilic characteristics. For certain purposes, for example, reinforcing agents for silicon rubber and other elastomers, it is desirable to have fillers which are hydrophobic and distinctly organophilic, but which also have an affinity for protons. The previously mentioned hydrophobic products do not possess this latter characteristic since the alkyl or aryl radicals, bonded to silicon atoms, possess no affinity for protons. Also, small residues of, for example, silanol groups, on the oxide particles (such as occur in a truly hydrophobic product) are screened by large numbers of alkyl groups in such a way that the particle does not exhibit proton affinity. This latter effect has actually become a quality characteristic of such products.
Clearly, entirely hydrophobic oxides are not wetted by water. However, finely divided hydrophobic silicon dioxide particles can demonstrate proton affinity by treating the particles in such a way that only part of the silanol groups located on the surface are converted to silicon alkyl groups, while the remainder of the silanol groups are converted to, for example, silicon alkoxy groups. Unlike the silicon alkyl group, the silicon alkoxy group exhibits proton affinity to a certain degree.
It is known in the art that such products can be produced by treating silicon dioxide with organoalkoxy silanes. The quantity ratio of silicon alkyl groups to silicon alkoxy groups located on the surface of the particles is equal to or greater than 1.
Particles, such as fillers, which are to be lipophilic, but which on the other hand are to possess a high degree of proton affinity with simultaneous nonwettability by water, must have as many silicon alkoxy groups as possible, and as few silanol groups as possible. Preferably the particles contain no silanol groups at all. The number of silocon alkyl groups is only of secondary importance, and in some cases of no importance at all. There presently exists a gap in the state of the art by which such products can be obtained.
It is known in the art that silicic acid aerogels can be given organophilic characteristics by coating them with a surface film of a polymeric organic silicate. (See DAS No. 1,048,889). These products are defined in part by the degree of polymerization of the surface film. If the degree of polymerization of the surface film lies above 25, the product can be hydrophilic, partially hydrophobic, but not organophilic or hydrophobic and organophilic. Since one is not dealing with uniformly hydrophobic material, but rather with hydrophilic particles wrapped in an organic more or less water repellant, layer wettable by water, considerable quantities of organic "wrapping material" are needed. For example, in order to obtain only partially hydrophobic products, about 20% by weight tetraethyl-alcosilicate is required. It will be apparant that in order to achieve completely hydrophobic products, disproportionately larger quantities of "wrapping material" are needed. For this reason, the production of entirely hydrophobic products by this method is impractical. (See the above DAS)